What is the size? Do you mean T>130 K shows positive magnetization? Then it means paramagnetic curie temperature of your material is 130 K. Does it shows hysteresis behavior at room temperature? If it does then what is Msat value. Does Msat decreases/increases with decrease in temperature? What about FC measurements? All these answers will clearly make you to understand the reason.
Dear Sridharan sir,
Crystallite size 39 nm, Tc=340K, Msat=17 emu/g, above 130K magnetization is +ve, and shows hyseresis behavior at room temperature.
Ferromagnetic material with Msat of 17 emu/g showing negative magnetization below 130K seems to be measurement artifact for me. Repeat your measurement for various field again and see what happens. Have you obtained the structure for chromium substituted cobalt ferrite through XRD? To my knowledge this type of material never produces the result described by you. It is better repeat the experiment also taking consideration of Sir Kai's answer.
Its due reverse spin of magnetic ions and particle size . Pls refer SmFeO3, NdFeO3 materials related papers.
@ramesh The materials mentioned by you has AFM behavior. Due to the canted AFM, it shows negative magnetization at low temperature. But ferrites with spinel structure are ferrimagnetic which are different.
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